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Showing papers on "Wave height published in 1997"


Journal ArticleDOI
TL;DR: In this paper, a series of time-averaged images of the swash zone are used to map contour lines of the beach surface, which are identified using bands of maximum brightness associated with breaking waves at the shoreline.

202 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach were investigated using field observations and numerical model predictions.
Abstract: Field observations and numerical model predictions are used to investigate the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach. Nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic (about 0.8-m significant wave height), narrowband swell propagated without breaking across an 80-m-wide, nearly flat (2-m depth) section of beach between a small offshore sand bar and a steep (slope = 0.1) beach face, where the waves finally broke. These nonlinear energy transfers are accurately predicted by a model based on the nondissipative, unidirectional (i.e., reflection is neglected) Boussinesq equations. For a lower-energy (wave height about 0.4 m) bimodal wave field, high-frequency seas dissipated in the surf zone, but lower-frequency swell partially reflected from the steep beach face, resulting in significant cross-shore modulation of swell energy. The combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations. Boussinesq model predictions on the flat section (where dissipation is weak) are improved by decomposing the wave field into seaward and shoreward propagating components. In more energetic (wave heights greater than 1 m) conditions, reflection is negligible, and the region of significant dissipation can extend well seaward of the sand bar. Differences between observed decreases in spectral levels and Boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum of breaking wave induced dissipation between adjacent measurement locations. The inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates.

125 citations


Journal ArticleDOI
TL;DR: In this article, the present status of ocean wave modeling at the European Centre for Medium-Range Weather Forecasts (ECMWF) is reviewed, and the analyzed wave height and peak period field are verified against buoy data and show a considerable improvement compared to verification results of a decade ago.
Abstract: The present status of ocean wave modeling at the European Centre for Medium-Range Weather Forecasts (ECMWF) is reviewed. Ocean waves are forecasted globally up to 10 days by means of the Wave Model (WAM), which is driven by 10-m winds from the ECMWF atmospheric model. Initial conditions are provided by assimilation of ERS-1 data into the first-guess wave field. The analyzed wave height and peak period field are verified against buoy data and show a considerable improvement compared to verification results of a decade ago. This is confirmed by a comparison of first-guess wave height against ERS-1 altimeter data. The main reasons for this improvement are (i) the higher quality of ECMWF winds compared to a decade ago, (ii) the improved physics of the WAM model, and (iii) the assimilation of ERS-1 data. The forecast skill of the ECMWF wave forecasting system is also studied by comparing forecasts with buoy data and verifying analysis. Error growth in forecast wave height is less rapid than in forecas...

108 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand.

95 citations


Journal ArticleDOI
TL;DR: In this article, a wave flume was used to study the differences between harmonic evolution of monochromatic waves as they propagate over a submerged impermeable or porous step under non-breaking conditions.

88 citations


Journal ArticleDOI
TL;DR: In this article, a new experimental investigation is presented which considers the hydrodynamics of the inner surf zone and swash zone using vertical wave gauges and a run-up wire.

69 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the almost-highest wave is unstable to infinitesimal disturbances, and the fastest growing instability is traced by a boundary-integral time-stepping method.
Abstract: The ‘almost-highest wave’, which is the asymptotic form of the flow in a steep irrotational water wave of less than the limiting height, was recently shown to be unstable to infinitesimal disturbances (see Longuet-Higgins & Cleaver 1994). It was also shown numerically that the lowest eigenfrequency is asymptotic to that of the lower superharmonic instability of a progressive wave in deep water (Longuet-Higgins, Cleaver & Fox 1994). In the present paper these calculations are revised, indicating the presence of more than one such instability, in agreement with recent calculations on steep periodic and solitary waves (Longuet-Higgins & Tanaka 1997).The nonlinear development of the fastest-growing instability is also traced by a boundary-integral time-stepping method and the initial, linear growth rate is confirmed. The subsequent, nonlinear stages of growth depend as expected on the sign of the initial perturbation. Perturbations of one sign lead to the familiar overturning of the wave crest. Perturbations of the opposite sign lead to a smooth transition of the wave to a lower progressive wave having nearly the same total energy, followed by a return to a wave of almost the initial wave height. This appears to be the beginning of a nonlinear recurrence phenomenon.

55 citations


Journal ArticleDOI
TL;DR: In this article, a modified non-linear cross-shore sediment transport relationship is developed based on equilibrium beach profile concepts and scaling relationships, which provides a reasonable explanation for the significantly different time scales of beach evolution evident in various laboratory experiments.

48 citations


DOI
05 Aug 1997
TL;DR: In this paper, the wave energy dissipation after breaking has been investigated with a number of random wave tests with a general formulation of stable wave height due to the random wave breaking.
Abstract: The process of wave energy dissipation after breaking has been investigated with a number of random wave tests. To obtain the data for wave breaking and its deformation, experiments have been conducted by utilizing a horizontal step adjoining to a combined slope of 1/20 and 1/10. After breaking, the wave height decreases by dissipation but attains a certain value at some distance from the breaking point. Experimental results show that the stable wave height is not constant but affected considerably by the wave period. The study has yielded a general formulation of stable wave height due to the random wave breaking. A new one-dimensional random wave deformation model is proposed, being coupled with nonlinear shoaling coefficient formula before wave breaking and the new energy dissipation term after breaking. The model is compared with the experimental data, large wave tank data, and field data. It predicts well the wave height deformation and the change of mean water level before and after wave breaking on arbitrary bottom profiles.

38 citations


Journal ArticleDOI
TL;DR: In this paper, the authors explore the impact of this undersampling on the calculated 100-yr wave height by running Monte Carlo simulations of simplified but realistic storms sampled by a simulated satellite and site.
Abstract: Since 1986, nine years of wave data derived from satellites have been accumulated, and this database will expand dramatically in the next two years as two more satellites are added. Several researchers have begun using this data to estimate extreme value statistics for waves. However, one potential problem with satellite data is space–time resolution, which is a poor match for the scales of storms. Satellites only revisit a site once every 10–35 days, and their tracks are separated by 100–200 km. With this coarse sampling, the satellite may miss storms since they have characteristic length and time scales as short as a few hours and tens of kilometers. The purpose of this paper is to explore the impact of this undersampling on the calculated 100-yr wave height. This is accomplished by running Monte Carlo simulations of simplified but realistic storms sampled by a simulated satellite and site. The authors study the sensitivity of the calculated 100-yr wave to variations in storm type, radius, and ...

37 citations


DOI
05 Aug 1997
TL;DR: In this article, two independent approaches are discussed whereby inverse modelling is employed as a means of better quantifying roller contributions in the near shore, and an intercomparison between the two approaches is made to get an indication of the validity of the applied theories and inverse modelling techniques.
Abstract: Two independent approaches are discussed whereby inverse modelling is employed as a means of better quantifying roller contributions in the near shore. A so-called integral approach utilizes a coupling between the extended wave energy and momentum balance equations to make inferences regarding roller properties based on wave height and set up measurements. In a somewhat more indirect inverse modelling approach, as first discussed in Mocke et al. (1994), the roller properties are obtained from predicted vertical distributions of internal flow properties such as turbulent kinetic energy, suspended sediment concentrations and undertow velocities which are optimally fitted against measured profiles. An intercomparison between the two approaches is made to get an indication of the validity of the applied theories and inverse modelling techniques. The integral approach further serves to evaluate a new conceptual model for dissipation due to wave breaking.

Proceedings ArticleDOI
06 Oct 1997
TL;DR: In this paper, the authors present wave directional spectra obtained using a conventional broadband acoustic Doppler current profiler (ADCP) and compare their observations with data collected using the Field Research Facility's pressure gauge array at Duck North Carolina.
Abstract: The authors present wave directional spectra obtained using a conventional broadband acoustic Doppler current profiler (ADCP) and compare their observations with data collected using the Field Research Facility's pressure gauge array at Duck North Carolina. Spectral computations are based on the maximum likelihood method. Comparisons include frequency-height spectra, directional spectra and computed parameters including significant height, peak period and peak direction. The authors conclude that the ADCP data compare well with the observations by the pressure gauge array.

Journal ArticleDOI
TL;DR: In this paper, the authors measured wave height, set-up and currents around a detached breakwater on a 1 in 50 plane beach and subjected to regular unidirectional waves, random unideal waves and directional random waves.

Journal ArticleDOI
TL;DR: In this paper, a new method was developed to estimate the incident and reflected root-mean-square (RMS) wave heights from the measured RMS wave height and wave setup.
Abstract: The cross-shore variation of the local reflection coefficient R is examined under the assumptions of normally incident wind waves on beaches with alongshore uniform bathymetry. The existing three-gauge method is modified to estimate R for the sloping bottom. A new method is developed to estimate the incident and reflected root-mean-square (RMS) wave heights from the measured RMS wave height and wave setup. These methods are used to analyze six runs from the large-scale SUPERTANK data set. The estimated values of R are affected little by the formation of a bar and increase shoreward with the increased percentage of breaking waves. One set of the DELILAH field data on a barred beach is also analyzed using collocated pressure and velocity measurements. This data set confirms the negligible effect of the bar on the variation of R over the bar in the absence of wave-breaking on the bar crest. These crude estimates of R suggest that the reflected wind waves may not be negligibly small in comparison to the incid...

DOI
01 Jan 1997
TL;DR: Based on a large amount of published laboratory results, reliable model is developed for computing beach profiles under regular wave actions as mentioned in this paper, where sediment transport is separated into suspended load and bed load, and the suspended load is computed as the product of the time-averaged suspended concentration and the timeaveraged velocity.
Abstract: Based on a large amount of published laboratory results, reliable model is developed for computing beach profiles under regular wave actions. The sediment transport is separated into suspended load and bed load. The suspended load is computed as the product of the time-averaged suspended concentration and the timeaveraged velocity. The bed load is developed following the similar step as Watanabe (1983) but the applied area is different. The wave model of Dally et al. (1985) is modified and used to compute wave height transformation. The beach profile change is computed from the conservation of sediment mass. The beach deformation model is verified with small scale and large scale experiments. Reasonably good agreement is obtained between measured and computed beach profiles.

Journal ArticleDOI
TL;DR: In this paper, the maximum stable wave height, compatible with the depth, is approached from above, while in bottom up environments, it is approaching from below, and the analyses of field data from these two environments are compared.


DOI
05 Aug 1997
TL;DR: In this article, a wave-by-wave model for wave height and the fraction of breaking waves was developed, in which the models employ a wave by-wave approach to calculate the shoaling, breaking and reforming of individual waves.
Abstract: Models for wave height and the fraction of breaking waves were developed; the models employ a wave-by-wave approach, in which the shoaling, breaking and reforming of individual waves are calculated. The performance of the models calibrated with experimental data was not satisfied; the fractions of breaking waves estimated by the models were smaller than the values measured over troughs in the field. The models therefore were calibrated and verified with the field data. Furthermore, the validity of the models calibrated with the field data was confirmed by comparison with large-scale experiment data.

Journal ArticleDOI
TL;DR: In this article, a theoretical model to examine oblique wave diffraction by a detached breakwater system consisting of an infinite row of regularly-spaced thin, impermeable structures located in water of uniform depth is presented.

Journal ArticleDOI
TL;DR: In this article, an analysis of the extreme statistical properties of these waves has been made based on more than 12 years of wave records from the Maersk Olie og Gas AS operated Gorm Field in the Danish sector of the Central North Sea.
Abstract: The area of the Central North Sea is notorious for the occurrence of very high waves in certain wave trains. The short-term distribution of these wave trains includes waves which are far steeper than predicted by the Rayleigh distribution. Such waves are often termed extreme waves or freak waves. An analysis of the extreme statistical properties of these waves has been made. The analysis is based on more than 12 yr of wave records from the Maersk Olie og Gas AS operated Gorm Field which is located in the Danish sector of the Central North Sea. From the wave recordings more than 400 freak wave candidates were found. The ratio between the extreme crest height and the significant wave height (20-min value) has been found to be about 1.8, and the ratio between extreme crest height and extreme wave height has been found to be 0.69. The latter ratio is clearly outside the range of Gaussian waves, and it is higher than the maximum value for steep nonlinear long-crested waves, thus indicating that freak waves are not of a permanent form, and probably of short-crested nature. The extreme statistical distribution is represented by a Weibull distribution with an upper bound,more » where the upper bound is the value for a depth-limited breaking wave. Based on the measured data, a procedure for determining the freak wave crest height with a given return period is proposed. A sensitivity analysis of the extreme value of the crest height is also made.« less

DOI
05 Aug 1997
TL;DR: In this paper, both the univariate and joint distributions of the significant wave height (Hs) and the zero-crossing wave period (Tz) were studied using 16 years of hourly wave data measured from five National Data Buoy Center (NDBC) buoy stations.
Abstract: Both the univariate and joint distributions of the significant wave height (Hs) and the zero-crossing wave period (Tz) are studied using 16 years of hourly wave data measured from five National Data Buoy Center (NDBC) buoy stations. For the univariate distribution, the log-normal distribution fits both the Hs and the Tz well. The modified log-normal distribution proposed by Fang and Hogben (1982) does improve the fit at the high Hs end and at the peak for some stations. For Hs, the Weibull distribution, with parameters computed from the maximum likelihood (ML) method, fits the upper tail of the cumulative distribution; however, it underpredicts both the probability peak and the probability density at the high end. For the joint distribution, the marginal Weibull/conditional log-normal distribution best describes the measured data of the steeper sea states and has the best overall fit.

Journal ArticleDOI
TL;DR: In this paper, the authors used data from the Surface Waves and Processes Program (SWAPP) to test current sea-surface drag parameterizations in open ocean conditions and found that the dependence of the drag coefficient on each of these parameters varies markedly from event to event.
Abstract: Data from the Surface Waves and Processes Program (SWAPP) are employed to test current sea-surface drag parameterizations in open ocean conditions. General trends in the data indicate that drag increases with increasing wind speed and wave height, and decreases with wave age. However, scatter in the data limits the use of these parameters and other wave dependent parameterizations for modelling efforts. Upon close inspection, it is found that during the onset of three wind events analyzed separately, each of these parameters correlate well with the drag coefficient. However, the dependence of the drag coefficient on each of these parameters varies markedly from event to event. The disparity appears most closely linked to the turning rate of the wind, indicating that temporal and directional effects may play an important role. A temporal lag of O(4) hours between the rise of the wind and subsequent rise in the drag coefficient is also noticed, further pointing out the complexity of the wind-stress system.

Journal ArticleDOI
01 Jul 1997
TL;DR: In this paper, a broad-band hydrophone was used to record ambient noise in the surf zone, in the frequency range 120 Hz to 5 kHz, in order to detect breaking waves.
Abstract: Ambient noise in the surf zone, in the frequency range 120 Hz to 5 kHz, was recorded using a broad-band hydrophone, located approximately 1 m above bottom and 1-2 m below the mean sea surface. The predominant source of this noise is breaking waves. Analysis of simultaneous land-based video observations of the sea surface in the region of the hydrophone, along with wave height data, reveals quantitative correlation between wave-breaking events and the hydrophone signal. In energetic surf, locally breaking waves appear as discrete events in the ambient noise spectra. Distant breaking events do not appear to be detected, as distinct events above the ambient background noise, by the hydrophone. The noise events associated with local breakers are characterized by an asymmetry in the time envelope: low frequencies (less than 500 Hz) are observed leading the breaking crest, followed by a broader range of frequencies peaking in intensity with the passage of the wave crest above the hydrophone, and then decreasing abruptly at all frequencies. Low frequencies are generally not observed trailing the breaking wave. The detection by the hydrophone of breaking waves in the immediate vicinity implies that ambient noise in heavy surf provides a means of studying breaking-wave statistics in the surf zone in situ: in particular, the frequency of occurrence of local breaking.

DOI
05 Aug 1997
TL;DR: In this article, a mild, impermeable slope was used to create a wide swash zone, and the long waves were found to be partially reflected from the swash zones and this partial reflection was successfully simulated by the superposition of an absorbed and a reflected long wave.
Abstract: This paper presents an analysis of physical model tests of the swash zone. A mild, impermeable slope was used to create a wide swash zone. Bichromatic and irregular waves with varying wave heights, wave periods and wave grouping were tested. The experiments showed the swash zone to be dominated by low frequency motions, which were closely related to the incident wave groups. The incident waves were still grouped in the surf and swash zones. An empirical relationship was determined, relating the long wave height at the shoreline to the deep water wave parameters. The long waves were found to be partially reflected from the swash zone and this partial reflection was successfully simulated by the superposition of an absorbed and a reflected long wave. The generation of spurious long waves and the seiche at the natural frequency of the equipment were also investigated.

DOI
05 Aug 1997
TL;DR: In this paper, a simple design expression was developed by introducing a depth modification into a standard depth-limited design equation, which resulted in an increase in the depth at the structure.
Abstract: Traditional depth-limited criteria for design of structures in shallow water were found not to be valid in hydraulic model tests with irregular waves. An extensive series of experiments showed that in addition to the height of the short period waves and the depth at the structure, the design wave height is a function of long wave action and wave setup at the structure. The additional effects are related to incident wave height and they result in an increase in depth at the structure. Thus, a simple design expression was developed by introducing a depth modification into a standard depth-limited design equation.

DOI
05 Aug 1997
TL;DR: In this article, the characteristics and occurrence condition of a new type breaker were investigated in a wave flume with 16 wave gages and numerical simulations using a fully nonlinear BIM (Boundary Integral Method) in a super-computer.
Abstract: Experiments in a wave flume(lm x 2m x 65m) with 16 wave gages and numerical simulations using a fully nonlinear BIM(Boundary Integral Method) in a super-computer(VP2600) are conducted to make clear the characteristics and occurrence condition of a new type breaker. Further, its deformation and wave height decay after breaking are investigated. It is shown that the new type breaker forms a giant jet with the size exceeding three times of the maximum jet size of usual type breakers and therefore is very efficient to wave control because of strong turbulence excited by plunging of the giant jet and the resultant remarkable absorption.

DOI
05 Aug 1997
TL;DR: In this paper, a system for estimating extremes of storm surge height and wave height associated with typhoons is presented, which consists of a Monte-Carlo simulation model for probabilistic generation of typhoon characteristics, termed a stochastic typhoon model, a parametric Typhoon model for wind estimation, numerical models for the computation of storm surges or waves and an extreme analysis model for estimation of their return values.
Abstract: This paper presents a system for estimating extremes of storm surge height and wave height associated with typhoons, which consists of a Monte-Carlo simulation model for probabilistic generation of typhoon characteristics, termed a stochastic typhoon model, a parametric typhoon model for wind estimation, numerical models for the computation of storm surges or waves and an extreme analysis model for estimation of their return values. The system is applied to the estimation of 50 to 1000-year return values and their confidence intervals based on the computations over a period of 1000 years for storm surge heights in Ise Bay connecting to the Pacific Ocean, and for wave heights at some representative locations around the Pacific coast of Japan.

DOI
05 Aug 1997
TL;DR: In this article, a Monte-Carlo simulation technique for the parent probability distributions of extreme wave height data was used to determine a preferable parameter estimation method for each of the parent distributions.
Abstract: Based on statistical analysis of extreme wave height data generated with a Monte-Carlo simulation technique for the prescribed parent probability distributions, a preferable method for the parameter estimation was determined for each of 8 distributions. It is also verified that a jackknife method is applicable to the correction of bias and the estimation of variance irrespective of parameter estimation method in most parent distributions, and that the information matrix methods inherent to the maximum likelihood method give generally satisfactory results in the estimation of variance of return wave height for samples of size greater than around 50.

Journal ArticleDOI
TL;DR: In this article, a series of waveforms as Topex transits a major typhoon is used to illustrate the problem and indicate how height, significant wave height, and backscatter are all affected.
Abstract: Atmospheric liquid water, occurring both as cloud and rain, is known to severely affect the performance of normal Ku-band altimeters. Examination of a series of waveforms as Topex transits a major typhoon is used to illustrate the problem and indicate how height, significant wave height, and backscatter are all affected. This behavior is contrasted with that at C band, where distortion of the waveform is minor as that frequency is far less susceptible to atmospheric attenuation. Algorithms are given for the maintenance of accurate wind speed and wave height measurements across such storms.

Proceedings ArticleDOI
03 Aug 1997
TL;DR: In this article, the authors performed some validation tests of a model, L. B. Wetzel (1990), which theoretically related the statistics of the ratio of illuminated to shadowed areas through the viewing geometry to the significant height of the waves causing the shadowing.
Abstract: When waves on the ocean surface are viewed with a ship-borne marine radar, their height is not directly related to either the magnitude of the microwave backscatter field or the magnitude of the modulation of this field in this grazing incidence regime. The authors have performed some validation tests of a model, L. B. Wetzel (1990), which theoretically related the statistics of the ratio of illuminated to shadowed areas through the viewing geometry to the significant height of the waves causing the shadowing. Using a subset of the data collected by two different digital marine radars on the Sea Truth and Radar Systems Experiment of December, 1994, the present authors calibrated the model against measured wave heights, and then validated it using the remaining data from the radars and other independent wave height estimations. Significant wave heights in these experiments ranged from less than one to greater than 5 metres. They found reasonable agreement between their inferred wave height extracted from radar imagery and heights estimated by other means when the sea conditions were moderate and not too complex. Scatter in the results was not inconsistent with the statistical variability expected from the sampling statistics. The model failed under conditions of crossing seas, where it overpredicted the wave height, and when the measured wave height was a significant fraction of the radar scanner height, where the model exhibited asymptotic behaviour and underpredicted the actual wave height.